Image forming system

ABSTRACT

An image forming system is disclosed which can prevent toner stain adhered onto an image bearing member at the time of switching from one developing unit to another from adhering to an image area on an intermediate transfer member and which can thereby prevent the deterioration of image quality. Latent images formed on a photoreceptor drum are developed by developing units in a revolver developing unit, toner images thus formed are primarily transferred in an overlapped state onto an intermediate transfer belt in a primary transfer section, and the thus primarily transferred toner images are together transferred secondarily onto a transfer paper. In this image forming system, a revolving operation of the revolver developing unit is controlled so that an area on the photoreceptor drum with which a developer contained in a Bk developing unit comes into contact upon 22.5° revolution of the revolver developing unit from a home position with consequent movement of the Bk developing unit to a developing position contacts a not-to-be-transferred area on the intermediate transfer belt in the primary transfer section. By so doing, toner stain caused by the Bk developing unit which has moved to the developing position does not exert any influence on the toner images formed on the intermediate transfer belt.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming system such as acopying machine, a facsimile, or a printer and more particularly to animage forming system wherein an image bearing member and an intermediatetransfer member are kept in contact with each other, toner images formedon the image bearing member are primarily transferred onto theintermediate transfer member, and the toner images thus primarilytransferred onto the intermediate transfer member are togethertransferred secondarily onto a transfer medium.

2. Description of the Prior Art

Heretofore, as this type of an image forming system, there is known animage forming system wherein toner images formed on a photoreceptor asan image bearing member are primarily transferred onto an intermediatetransfer member by utilizing an electrostatic force and thereafter thetoner images on the intermediate transfer member are secondarilytransferred onto a transfer paper by utilizing an electrostatic force.The image forming system using such an intermediate transfer member isadvantageous in that images can be formed on various kinds of transferpapers, including plain paper and cardboard.

Among image forming systems having an intermediate transfer member,there is known one which is constructed so as to perform image formationin a constantly contacted state of the intermediate transfer member witha photoreceptor. This image forming system does not require theprovision of an engaging/disengaging mechanism for the engagement anddisengagement of the intermediate transfer medium with respect to thephotoreceptor. Thus, the image forming system in question isadvantageous in that the components' cost can be reduced by an amountcorresponding to the disengaging mechanism and hence space-saving can somuch be attained in comparison with an image forming system which has aconstruction permitting engagement and disengagement of the intermediatetransfer member with and from the photoreceptor.

Moreover, in the image forming system having an intermediate transfermember, toner images formed on a photoreceptor can be overlapped ontothe intermediate transfer member, thus permitting the image formingsystem to be widely used as a color image forming system capable offorming a color image on a transfer paper. In the color image formingsystem there is used a developing apparatus having plural developingunits capable of effecting development using different colors ofdevelopers, and latent images are developed by corresponding developingunits respectively. In such an image forming system, when latent imagesformed successively on a single photoreceptor are developed bycorresponding developing units respectively, there arises the necessityof switching from one to another developing unit in a successive manner.In connection with this developing unit switching operation, there areknown a method wherein one and same developing position is used for thedeveloping units and the developing units themselves are movedsuccessively for development to the developing position and a methodwherein different developing positions are used for the developing unitsrespectively and the developing units themselves do not move. Asexamples of the former method are mentioned a revolver method and aslider method. On the other hand, as an example of the latter methodthere is known a method wherein switching is made from one to anotherdeveloping unit by bringing only the developer in the developing unit tobe used into contact with a photoreceptor with use of anengaging/disengaging mechanism provided in each developing unit.

In a developing apparatus adopting a revolver method or a slide methodwherein developing units themselves are moved to a developing position,the developing units are generally arranged so that they can move to thedeveloping position successively in accordance with a development orderadopted in forming a full-color image in which image formation isperformed using all the developing units. For example, in a revolverdeveloping apparatus adopting a revolver method wherein development isperformed in the order of black (“Bk” hereinafter), yellow (“Y”), cyan(“C”), and magenta (“M”) at the time of forming a full-color image,these four-color developing units are arranged side by side in theirrevolving direction in the order of Bk, Y, C, and M.

In the image forming system provided with such a revolver developingapparatus, the developing unit which is the first to make developmentmust lie in the developing position before the start of development. Formoving each developing unit most efficiently at the time of forming afull-color image, the revolver developing apparatus, before the start ofthe image forming process, is stopped at a home position where the Bkdeveloping unit which is the first to perform development is locatednearest to an upstream side in the revolving direction of the revolverdeveloping apparatus. From this home position the developing unit whichis used in this image forming process and which is the first to makedevelopment is moved to the developing position to effect development.

However, when the image forming process is started and the developingunit which is the first to make development has moved to the developingposition, the developer in the developing unit adheres onto aphotoreceptor already before the start of development, causing stain ofthe toner present on the photoreceptor. The first reason for theoccurrence of such toner stain is presumed to be as follows. When thedeveloper pressure increases between a developer carrier in a developingunit and a photoreceptor in relation to both a development gap and adeveloper scoop-up quantity (developer weight per unit area), thereincreases an impact force between the developer carrier and thephotoreceptor with movement of the developing unit, so that it becomeseasier for the toner to leave the developer carrier, and the spilt toneradheres onto the photoreceptor with van der Waals' force. The secondreason is presumed to be as follows. The potential of a toner layerportion deposited on the developer carrier approaches a surfacepotential of the photoreceptor rather than the potential of thedeveloper carrier surface, causing an effective bias of development tobe changed, with consequent deposition of toner onto the photoreceptor.The toner stain thus generated is conveyed to a primary transfer sectionas it is adhered to the photoreceptor and adheres onto an intermediatetransfer member. If the surface portion of the intermediate transfermember thus stained with the toner stain is an area (a“to-be-transferred area” hereinafter) onto which a toner image on thephotoreceptor is to be primarily transferred in this image formingprocess, the toner stain overlaps a toner image to be subsequentlytransferred primarily onto the to-be-transferred area. The resultingground stain causes deterioration of the image quality.

In the case where image formation is to be done using the above imageforming system and using, for example, only the three colors of Y, C,and M without using Bk, it is necessary that the Y developing unit whichis the first to make development be moved to the developing positionafter start of the image forming process. In this case, it is necessarythat the Y developing unit be moved to the developing position whileskipping over the Bk developing unit from the home position. During thismovement, the developer in the Bk developing unit comes into contactwith the photoreceptor surface at the developing position. Also at theinstant of this contact the toner adheres onto the photoreceptor for thesame reason as above and the resulting toner stain adheres onto theintermediate transfer member. If this stained portion is theto-be-transferred area, a ground stain results and causes an imagequality deterioration like above.

In the case where image formation is to be performed using the aboveimage forming system and using, for example, only the two colors of Bkand M, it is necessary that, after the completion of development by theBk developing unit, the M developing unit to be used next fordevelopment be moved to the developing position. In this case, if afull-color image is to be formed, it is necessary that the M developingunit be moved to the developing position while skipping over the Ydeveloping unit to be next used for development and further skippingover the C developing unit to be used for development next to the Ydeveloping unit. At this time, if the to-be-transferred area in thesurface movement direction of the intermediate transfer member is long,a not-to-be-transferred area of the intermediate transfer member becomesvery narrow. Therefore, in relation to the developing unit moving time,even if the movement of the M developing unit is started just after thecompletion of Bk development, there sometimes occurs a case where the Mdeveloping unit cannot be moved to the developing position before afront end of the to-be-transferred area on the intermediate transfermember reaches the primary transfer section. In this case, M toner imagecannot be primarily transferred onto the intermediate transfer member,so there arises the necessity of causing the intermediate transfermember to idle-rotate and thereby causing the to-be-transferred area toagain reach the primary transfer section. During this idle-rotation,since the to-be-transferred area on the intermediate transfer member islong, C and M toner stains adhere to the to-be-transferred area on theintermediate transfer member, with consequent ground stain causing thedeterioration of image quality.

Reference will be made below to a concrete example of image formationperformed using an image forming system and using only the two colors ofBk and M. In the image forming apparatus used, the time required forswitching from one to another developing unit in a revolver developingapparatus (the time required for 90° revolution) was 320 msec and acircumferential length of an intermediate transfer belt as anintermediate transfer member was 565.5 mm.

FIGS. 10(a) to 10(d) are timing charts showing a part of a conventionalsequence control operation performed when an image is formed in alongitudinal direction of Japanese Industrial Standard A3-size paper (adirection in which the longitudinal direction of the paper is positionedin parallel with a surface movement direction on an intermediatetransfer belt). FIG. 10(e) is a timing chart showing at what timing ato-be-transferred area on the intermediate transfer belt passes aprimary transfer section. As shown in FIG. 10(e), Y toner stain and Ctoner stain adhere to a not-to-be-transferred area on the intermediatetransfer belt, but M toner stain adheres within the to-be-transferredarea. As a result, a lateral band-like ground stain attributable to Mtoner stain occurred in a portion 21 mm from the image front end on thepaper.

When a 12×18 in. paper image larger than the Japanese IndustrialStandard A3-size paper image was formed, C and M toner stains adhered tothe to-be-transferred area on the intermediate transfer member and thereoccurred a lateral band-like ground stain attributable to the C tonerstain at a portion 24 mm from the front end portion of the image on thepaper and a lateral band-like ground stain attributable to the M tonerstain at a portion 63 mm from the image front end. Further, when animage in the transverse direction of a Japanese Industrial StandardA4-size paper was formed by double-sheet image formation onto anintermediate transfer belt, there occurred a lateral band-like groundstain attributable to C toner stain at a portion 13 mm from the imagefront end on paper corresponding to the first sheet of image on theintermediate transfer belt and there occurred a lateral band-like groundstain attributable to M toner stain at a portion 50 mm from the imagefront end on the paper.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of theabove-mentioned background and it is an object of the invention toprovide an image forming system capable of preventing a toner stainadhered onto an image bearing member from adhering to ato-be-transferred area on an intermediate transfer member.

For achieving the above-mentioned object, in a first aspect of thepresent invention, there is provided an image forming system comprisingan image bearing member, an intermediate transfer member which is keptin contact with the image bearing member, a developing apparatus havingplural developing units, the developing apparatus causing a developercontained in a predetermined one of the developing units into contactwith the image bearing member to develop a latent image formed on theimage bearing member, a developing unit moving means capable of movingthe predetermined developing unit to a developing position where thedeveloper in the predetermined developing unit comes into contact withthe image bearing member, latent images formed on the image bearingmember being developed respectively by the developing units of thedeveloping apparatus, toner images thus formed on the image bearingmember being primarily transferred onto the intermediate transfer memberin a primary transfer section in which the image bearing member and theintermediate transfer member are in contact with each other, and thetoner images thus primarily transferred onto the intermediate transfermember being together transferred secondarily onto a transfer medium,and a control means which controls the developing unit moving means insuch a manner that an area on the image bearing member at which thedeveloper contained in the predetermined developing unit contacts theimage bearing member to effect development, comes to contact anot-to-be-transferred area on the intermediate transfer member in theprimary transfer section.

The “not-to-be-transferred area” as referred to herein indicates an areaon an intermediate transfer belt onto which a toner image formed on aphotoreceptor is not primarily transferred.

According to this image forming system, toner stain adhered to the imagebearing member can be prevented from adhering to the to-be-transferredarea on the intermediate transfer member which adhesion is caused by,for example, an impart force induced upon movement of the developingunit to effect development to the developing position.

In second to sixth aspects of the present invention there is provided animage forming system comprising an image bearing member, an intermediatetransfer member which is kept in contact with the image bearing member,a developing apparatus having plural developing units, the developingapparatus causing a developer contained in a predetermined one of thedeveloping units into contact with the image bearing member to develop alatent image formed on the image bearing member, a developing unitmoving means capable of moving the predetermined developing unit to adeveloping position where the developer in the predetermined developingunit comes into contact with the image bearing member, latent imagesformed on the image bearing member being developed respectively by thedeveloping units of the developing apparatus, toner images formed on theimage bearing member being primarily transferred onto the intermediatetransfer member in a primary transfer section in which the image bearingmember and the intermediate transfer member are in contact with eachother, and the toner images thus primarily transferred onto theintermediate transfer member being together transferred secondarily ontoa transfer medium, and a control means which, when moving one of thepredetermined developing unit to effect development to the developingposition so that any of the other developing units positioned on anupstream side in a developing unit moving direction of the developingposition with respect to the predetermined developing unit passes thedeveloping position, controls the developing unit moving means in such amanner that an area on the image bearing member at which a developercontained in the any of the other developing units contacts the imagebearing member when the any of the other developing units passes thedeveloping position, is an area on the image bearing member which areacomes into contact with a not-to-be-transferred area on the intermediatetransfer member in the primary transfer section.

According to this image forming system, when moving the developing unitto effect development to the developing position while skipping over thedeveloping units not to effect development, a toner stain which adheresto the image bearing member when any of the developing units not toeffect development passes the developing position can be prevented fromadhering to the to-be-transferred area on the intermediate transfermember.

Particularly, in a third aspect of the present invention there isprovided, in combination with the above second aspect, an image formingsystem further comprising a control switching means which switches fromone method to another for controlling the developing unit moving meansin accordance with image forming conditions involving different lengthsof the not-to-be-transferred area in a surface movement direction of theintermediate transfer member, and wherein the control means controls thedeveloping unit control means in accordance with the control methodswitched by the control switching means.

In this image forming system, even if the length of thenot-to-be-transferred area in the surface movement direction of theintermediate transfer member changes according to image formingconditions in the image forming process, it is possible to let tonerimage adhere to the not-to-be-transferred area by switching to a controlmethod which matches the not-to-be-transferred area

In a fourth aspect of the present invention there is provided, incombination with the above third aspect, an image forming system whereinthe control switching means has a storage medium which stores pluralcontrol methods corresponding respectively to the above image formingconditions and also has a control method read means for reading from thestorage medium a control method corresponding to an image formingcondition for an image forming process carried out by the image formingsystem.

According to this image forming system, an appropriate control methodmatching the image forming condition adopted in the image formingprocess concerned can be read from the storage medium which pre-storescontrol methods corresponding to different lengths of thenot-to-be-transferred area.

In a fifth aspect of the present invention there is provided, incombination with the above second aspect, an image forming systemfurther comprising a control switching means for switching from onecontrol method to another to control the developing unit moving means inaccordance with the type of a developing unit used in an image formingprocess carried out by the image forming system, and wherein the controlmeans controls the developing unit moving means in accordance with thecontrol method switched by the control switching means.

In this image forming system, in case of carrying out the image formingprocess by using any of the plural developing units in the developingapparatus, there sometimes arises the necessity of moving the developingunit used to the developing position while skipping over the developingunits not used. In this case, by switching to the control method whichmatches the type of the developing unit used, toner stain adhered to theimage bearing member can be prevented from adhering to theto-be-transferred area on the intermediate transfer member when any ofthe developing units not used passes the developing position.

Further, in a sixth aspect of the present invention there is provided,in combination with the above fifth aspect, an image forming systemwherein the control switching means has a storage medium which storesplural control methods corresponding respectively to the types of thedeveloping units used in the image forming process and also has acontrol method read means for reading the control methods correspondingto the types of the developing units from the storage medium.

According to this image forming system, an appropriate control methodmatching the image forming condition adopted in the image formingprocess concerned can be read from the storage medium which pre-storescontrol methods corresponding to the types of the developing units used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(d) are timing charts showing a part of sequence controloperations performed by a control section related to Control Example 1in an embodiment of the present invention;

FIG. 1(e) is a timing chart showing at what timing a to-be-transferredarea on an intermediate transfer belt passes a primary transfer section,in a corresponding relation to the timing charts of FIGS. 1(a) to 1(d);

FIG. 2 is a schematic construction diagram of an entire copying machineembodying the present invention;

FIG. 3 is a schematic construction diagram of an image forming sectionas a principal section of the copying machine;

FIG. 4 is a schematic construction diagram of a revolver developingapparatus used in the copying machine;

FIGS. 5(a) to 5(d) are timing charts showing a part of sequence controloperations performed by a control section related to Control Example 2in the embodiment;

FIG. 5(e) is a timing chart showing at what timing the to-be-transferredarea on the intermediate transfer belt passes the primary transfersection, in a corresponding relation to the timing charts of FIGS. 5(a)to 5(d);

FIGS. 6(a) to 6(d) are timing charts showing a part of sequence controloperations performed by a control section related to Control Example 3in the embodiment;

FIG. 6(e) is a timing chart showing at what timing the to-be-transferredarea on the intermediate transfer belt passes the primary transfersection, in a corresponding relation to the timing charts of FIGS. 6(a)to 6(d);

FIGS. 7(a) to 7(d) are timing charts showing a part of sequence controloperations performed by a control section related to Control Example 4in the embodiment;

FIG. 7(e) is a timing chart showing at what timing the to-be-transferredarea on the intermediate transfer belt passes the primary transfersection, in a corresponding relation to the timing charts of FIGS. 7(a)to 7(d);

FIGS. 8(a) to 8(d) are timing charts showing a part of sequence controloperations performed by a control section related to Control Example 5in the embodiment;

FIG. 8(e) is a timing chart showing at what timing the to-be-transferredarea on the intermediate transfer belt passes the primary transfersection;

FIGS. 9(a) to 9(d) are timing charts showing a control method adopted incase of toner stain adhering to the to-be-transferred area on theintermediate transfer belt, for comparison with Control Example 1;

FIG. 9(e) is a timing chart showing at what timing the to-be-transferredarea on the intermediate transfer belt passes the primary transfersection, in a corresponding relation to the timing charts of FIGS. 9(a)to 9(d);

FIGS. 10(a) to 10(d) are timing charts showing a conventional controlmethod adopted in case of toner stain adhering to the to-be-transferredarea on the intermediate transfer belt; and

FIG. 10(e) is a timing chart showing at what timing theto-be-transferred area on the intermediate transfer belt passes theprimary transfer section, in a corresponding relation to the timingcharts of FIGS. 10(a) to 10(d).

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An embodiment of the present invention will be described hereinunder inwhich the invention is applied to an electrophotographic copying machine(hereinafter referred to simply as “copying machine”) as an imageforming system.

Reference will first be made to an entire construction of the copyingmachine of the embodiment.

FIG. 2 is a schematic construction diagram of the entire copying machineof the embodiment, FIG. 3 is a schematic construction diagram of animage forming section as a principal section of the copying machine, andFIG. 4 is a schematic construction diagram of a revolver developingapparatus used in the copying machine. The copying machine, indicated at1, is mainly composed of an image read section (“scanner section”hereinafter) 2, an image forming section (“printer section” hereinafter)3 shown in FIG. 3, and a paper feed cassette section 4.

The scanner section 2 reads color image information on an original 5 foreach of color separation lights of, for example, red, green, and blue(“R,” “G,” “B,” respectively) and converts the thus-read color imageinformation pieces into electrical image signals. Then, on the basis ofintensity levels of these R, G, B color separation image signals, acolor conversion processing is performed in an image processing section(not shown) to afford image data of Bk, Y, C, and M. The image data thusobtained are sent to the printer section 3.

The printer section 3 comprises a photoreceptor drum 200 as an imagebearing member, a charger 201 as a charging means, a destaticizer 202 asa photoreceptor destaticizing means, a photoreceptor cleaner 210consisting of a cleaning blade and a fur brush, an optical write unit220 as a latent image forming means, a revolver developing unit 400 as adeveloping apparatus, an intermediate transfer unit 500, a secondarytransfer unit 600 as a secondary transfer means, and a fixing unit 700as a fixing means using a pair of fixing rollers 701. The photoreceptordrum 200 rotates counterclockwise as indicated with arrow A in FIG. 3.Around the photoreceptor drum 200 are arranged the charger 201, thephotoreceptor cleaner 210, a developing unit selected from amongdeveloping units in the revolver developing unit 400, and theintermediate transfer unit 500.

The optical write unit 220 converts image data provided from the scannersection 2 into an optical signal and radiates laser beam L correspondingto an image on the original to the surface of the photoreceptor drum 200which is charged uniformly by the charger 201, to effect an opticalwrite and form an electrostatic latent image on the surface of thephotoreceptor drum. The optical write unit 220 may be composed of asemiconductor laser as a light source, a laser emission drivecontroller, a polygon mirror, a motor for rotating the polygon mirror,an f/θ lens, and a reflecting mirror.

As shown in FIG. 4, the revolver developing unit 400 comprises a Bkdeveloping unit 410 which uses Bk toner, a Y developing unit 420 whichuses Y toner, a C developing unit 430 which uses C toner, an Mdeveloping unit 440 which uses M toner, and a revolver drive unit. Thedeveloping units 410, 420, 430, and 440 as constituents of the revolverdeveloping unit 400 respectively comprise developing sleeves 411, 421,431, and 441 as developer carrier members, the developing sleeves 411,421, 431, and 441 being adapted to rotate while allowing the crest of adeveloper to be in contact with the surface of the photoreceptor drum200 for developing electrostatic latent images on the photoreceptordrum, developer paddles 412, 422, 432, and 442 adapted to rotate forscooping up and agitating the developer, and a sleeve drive section (notshown) for rotating the developing sleeves.

In this embodiment, toners contained in the developing units 410, 420,430, and 440 are fed from toner bottles 413, 423, 433, and 443,respectively, and are charged in negative polarity by agitation togetherwith a ferrite carrier. To the developing sleeves 411, 421, 431, and 441in the developing units is applied a developing bias from a power supplyserving as a developing bias application means (not shown), thedeveloping bias comprising an AC voltage (AC component) superposed on anegative DC voltage (DC component). As a result, a predetermineddeveloping bias is applied between each developing sleeve and a metallicbase layer formed on the photoreceptor drum 200.

When the copying machine 1 is in a stand-by state, as shown in FIG. 4,the revolver developing unit 400 causes the Bk developing unit 410 tostop at a home position with respect to a developing position. The homeposition in this embodiment is set at a 22.5°-revolved position of thedeveloping sleeve 411 in the Bk developing unit 410 on an upstream sidein a revolving direction indicated with arrow D in the figure withrespect to a developing position opposed to the photoreceptor drum 200.

The intermediate transfer unit 500 comprises an intermediate transferbelt 501 as an intermediate transfer member and plural rollers 507 to512 on which is stretched the intermediate transfer belt 501. Around andin opposition to the intermediate transfer belt 501 are arranged thesecond transfer unit 600, a belt cleaning blade 504 as an intermediatetransfer member cleaning means, a lubricant application brush 505 as alubricant application means, and an optical sensor 514 as a mark sensorfor detecting a position detecting mark put on the back of a non-imageforming area of the intermediate transfer belt. The intermediatetransfer belt 501 is entrained on a primary transfer bias roller 507 asa primary transfer means, a belt drive roller 508 as a belt drive means,a belt tension roller 509, a secondary transfer opposition roller 510, acleaning opposition roller 511, and an earth roller 512. These rollersare formed using an electrically conductive material and the otherrollers than the primary transfer bias roller 507 are connected toground. A primary transfer power supply 801, which is constant current-or voltage-controlled, is connected to the primary transfer bias roller507. By means of a control section as a control means which will bedescribed later, the primary transfer bias roller 507 is controlled to acurrent or voltage of a predetermined magnitude according to the numberof overlaps of toner images and is applied with a primary transfer biasat a predetermined application timing.

The intermediate transfer belt 501 is driven in the direction of arrow Bin FIG. 3 by the belt drive roller 508 which is rotated with a drivemotor (not shown). The intermediate transfer belt 501 has a single ormulti-layer structure formed of a semiconductor or an insulator. Itssurface resistance value is set at 10¹² Ω/cm2 or so, whereby it ispossible to prevent sneaking of a primary transfer electric field at thetime of primary transfer and hence possible to suppress the occurrenceof lap dirt.

In the primary transfer section where toner images of various colorsformed on the photoreceptor drum 200 are transferred onto theintermediate transfer belt 501, the intermediate transfer belt is pushedagainst the photoreceptor drum by means of the primary transfer biasroller 507 and the earth roller 512. As a result, in the primarytransfer section, a nip portion of a predetermined width is formedbetween the photoreceptor drum 200 and the intermediate transfer belt501.

The lubricant application brush 505 is for applying fine particles tothe intermediate transfer belt 501 which fine particles are obtained byscraping off a plate-like zinc stearate as lubricant. The lubricantapplication brush 505 is constituted so as to be movable into contactwith the intermediate transfer belt 501 at a predetermined timing andaway from the belt.

The secondary transfer unit 600 comprises a secondary transfer belt 601,three support rollers 602, 603, and 604 which support the secondarytransfer belt 601 in a stretched state, and a secondary transfer biasroller 605. A stretched portion of the secondary transfer belt 601positioned between the support rollers 602 and 603 can be put inpressure contact with the secondary transfer opposition roller 510through the intermediate transfer belt 501. One of the three supportrollers 602, 603, and 604 is a drive roller which is rotated by a drivemeans (not shown), and with this drive roller, the secondary transferbelt 601 is moved in the direction of arrow C in the figure.

The secondary transfer bias roller 605 is disposed in such a manner thatthe intermediate transfer belt 501 and the secondary transfer belt 601are sandwiched in between the secondary transfer bias roller and thesecondary transfer opposition roller 510 in the intermediate transferunit 500. A transfer bias of a predetermined current is applied to thesecondary transfer bias roller 605 by means of a secondary transferpower supply 802 which is constant current-controlled. In the secondarytransfer unit 600, the support roller 602 and the secondary transferbias roller 605 are each provided with a not-shown engaging/disengagingmechanism so that the secondary transfer belt 601 and the secondarytransfer bias roller 605 can move into contact with and away from thesecondary transfer opposition roller 510. A dash-double dot line in FIG.3 represents a spaced position of the secondary transfer belt 601 andthe support roller 602.

With a pair of resist rollers 650, transfer paper P as a transfer mediumis fed at a predetermined timing to a secondary transfer section formedbetween the intermediate transfer belt 501 and the secondary transferbelt 601 which are sandwiched in between the secondary transfer biasroller 605 and the secondary transfer opposition roller 510. At aportion of the secondary transfer belt 601 at which the secondarytransfer belt is entrained on the support roller 603 located on thefixing unit 700 side, a transfer paper destaticizing charger 606 as atransfer medium destaticizing means and a belt destaticizing charger 607as a transfer medium support member destaticizing means are opposed toeach other. Further, a cleaning blade 608 as a transfer medium supportmember cleaning means is in abutment against a portion of the secondarytransfer belt 601 which portion is entrained on the support roller 604.

The transfer paper destaticizing charger 606 destaticizes ah electriccharge held on the transfer paper P, thereby permitting the transferpaper to be separated in a satisfactory manner from the secondarytransfer belt 601 by virtue of a high stiffness of the transfer paperitself. The belt destaticizing charger 607 destaticizes an electriccharge remaining on the secondary transfer belt 601. The cleaning blade608 functions to remove deposits on the surface of the secondarytransfer belt 601 and thereby clean the said surface.

CONTROL EXAMPLE 1

Now, a description will be given below about one control example(“Control Example 1” hereinafter) of forming a full-color image with useof all the developing units 410, 420, 430, and 440. In this controlexample reference will be made to the case where an image is formed inthe longitudinal direction of Japanese Industrial Standard A3-sizepaper.

FIGS. 1(a) to 1(d) are timing charts showing a part of sequence controloperations performed by a control section as a control related to thiscontrol example and FIG. 1(e) is a timing chart showing at what timing ato-be-transferred area on the intermediate transfer belt 501 passes theprimary transfer section.

When a user pushes a Copy Start key, the photoreceptor dram 200 startsrotating in the direction of arrow A in FIG. 3 with a drive motor (notshown) and the intermediate transfer belt 501 rotates in the directionof arrow B with the belt drive roller 508 (FIG. 1(a)). When the positiondetecting mark put on the intermediate transfer belt 501 is detected bythe optical sensor 514 (FIG. 1(b)), an F gate signal is outputtedslightly later than the mark detection timing (FIG. 1(c)). While the Fgate signal is outputted, reading of image data from the original 5 isstarted and an optical write with laser beam L is performed on the basisof the image data. As a result, an electrostatic latent image of Bk isfirst formed on the photoreceptor drum 200.

In the revolver developing unit 400, before the Copy Start key ispushed, the Bk developing unit 410 assumes the home position which is a22.5°-revolved position upstream in the revolving direction indicatedwith arrow D in FIG. 4 with respect to the developing position. In thiscase, if the revolver developing unit 400 is revolved 22.5°, causing theBk developing unit 410 to move to the developing position, just afterdepression of the Copy Start key, then as shown in FIGS. 9(d) and 9(e),at the end of movement of the Bk developing unit, Bk toner stains in theBk developing unit adheres to the surface of the photoreceptor drum 200which comes into contact with the to-be-transferred area on theintermediate transfer belt 501. In the first transfer section this Bktoner stain adheres to the to-be-transferred area on the intermediatetransfer belt 501 and causes deterioration of the image quality.

In view of this point, in this control example, after the Copy Start keyis pushed and after the area on the photoreceptor drum 200 which comesinto contact with the to-be-transferred area on the intermediatetransfer belt 501 passes the developing position, the revolverdeveloping unit 400 is revolved, causing the Bk developing unit 410 tomove to the developing position (FIG. 1(d)). Consequently, as shown inFIG. 1(e), the Bk toner stain adheres to a not-to-be-transferred area onthe intermediate transfer belt 501, with no influence exerted on theimage quality.

The Bk developing unit 410 which has thus moved to the developingposition develops the electrostatic latent image of Bk arriving at thedeveloping position, whereby the Bk toner image formed on thephotoreceptor drum 200 is primarily transferred to the to-be-transferredarea on the intermediate transfer belt. Thereafter, the Y developingunit 420 which is to effect development next also moves so that thesurface of the photoreceptor drum 200 for contact therewith upon arrivalat the developing position comes into contact with anot-to-be-transferred area on the intermediate transfer belt 501. Then,Y toner image formed on the photoreceptor drum 200 by the Y developingunit 420 is primarily transferred so as to overlap the Bk toner image onthe intermediate transfer belt 501. This is also true of the subsequentC and M.

The toner images thus formed in an overlapped state of four colors onthe intermediate transfer belt 501 are then transferred together ontotransfer paper P in the secondary transfer section. The transfer paper Pis fed to the secondary transfer section by the paired resist rollers650 when the front end of toner image on the intermediate transfer belt501 comes to the second transfer section. At the beginning of the imageforming operation the transfer paper P is already fed to the pairedresist rollers 650 from a transfer paper cassette 6 in the paper feedcassette section 4 or from a manual paper feed tray 7. When the transferpaper P passes the secondary transfer section in an overlapped statewith the toner images on the intermediate transfer belt 501, the tonerimages are together transferred onto the transfer paper P with asecondary transfer bias applied to the secondary bias roller 605.

With movement of the secondary transfer belt 601, the transfer paper Pwith toner images thus transferred thereto is conveyed, and when passingthe portion opposed to the transfer paper destaticizing charger 606, thetransfer paper is destaticized and leaves the secondary transfer belt.Then, the toner images are melt-fixed in the nip portion of the pairedfixing rollers 701 and the transfer paper P is discharged to theexterior of the copying machine by means of a pair of discharge rollers8.

After the above primary transfer, the photoreceptor drum 200 isdestaticized uniformly by means of a destaticizer 202 and thereafterresidual toner remaining on the surface of the photoreceptor drum issubjected to cleaning by the photo receptor cleaner 210. Likewise,residual toner which is left unused on the surface of the intermediatetransfer belt 501 after the secondary transfer is subjected to cleaningby the belt cleaning belt 504 which is pushed against the intermediatetransfer belt 501 by an engaging/disengaging mechanism (not shown).

CONTROL EXAMPLE 2

The following description is now provided about a control example(“Control Example 2” hereinafter) wherein image formation is performedusing only Y developing unit 420, C developing unit 430, and Mdeveloping unit 440 without using Bk developing unit 410. Also in thiscontrol example, as in the above Control Example 1, an example will begiven in which image formation is performed in the longitudinaldirection of Japanese Industrial Standard A3-size paper.

FIGS. 5(a) to 5(d) are timing charts showing a part of sequence controloperations performed by a control section as a control means related tothis control example and FIG. 5(e) is a timing chart showing at whattiming the to-be-transferred area on the intermediate transfer belt 501passes the primary transfer section.

When the user pushes the Copy Start key, as in the previous ControlExample 1, the photoreceptor drum 200 and the intermediate transfer belt501 start rotating (FIG. 5(a)) and the position detecting mark on theintermediate transfer belt is detected by the optical sensor 514 FIG.5(b)), then an F gate signal is outputted slightly later than the markdetection timing (FIG. 5(c)). First, an electrostatic latent image of Yis formed on the photoreceptor drum 200.

After depression of the Copy Start key and before the front end portionof the electrostatic latent image of Y arrives at the developingposition, the revolver developing unit 400 which has stood by at thehome position revolves 90° to let the Y developing unit 420 stand by ata development stand-by position which is a 22.5°-revolved positionupstream in the revolving direction indicated with arrow D in FIG. 4with respect to the developing position. Upon this 22.5° revolution thedeveloper contained in the Bk developing unit 410 comes into contactwith the upper surface of the photoreceptor drum 200 in the developingposition. Therefore, if the revolver developing unit 400 is revolved 90°just after depression of the Copy Start key, Bk toner stain caused bythe aforesaid contact adheres to the to-be-transferred area on theintermediate transfer belt 501 as in the example illustrated in FIG. 9.

In this control example, the revolver developing unit 400 is revolved90° after depression of the Copy Start key and after the area on thephotoreceptor drum 200 which contacts the to-be-transferred area on theintermediate transfer belt 501 passes the developing position.Consequently, as shown in FIG. 5(e), Bk toner stain, when passing thedeveloping position, adheres to a not-to-be-transferred area on theintermediate transfer belt 501, exerting no influence on the imagequality.

When the 90° revolution is thus completed, the Y developing unit 420assumes the development stand-by position which is a 22.5° revolvedposition upstream in the revolving direction from the developingposition. Simultaneously with the arrival timing of the electrostaticlatent image of Y at the developing position the revolver developingunit 400 is revolved 22.5°, causing the Y developing unit 420 to move tothe developing position. Thus, by once allowing the Y developing unit420 to stand by at the development stand-by position, it is possible todiminish the time of contact with the photoreceptor drum 200 and hencepossible to prevent a waste consumption of toner adhered onto thephotoreceptor drum when the electrostatic latent image of Y is notdeveloped.

CONTROL EXAMPLE 3

Next, a description will be given about a control example (“ControlExample 3” hereinafter) wherein image formation is performed using onlyBk developing unit 410 and M developing unit 440 without using Ydeveloping unit 420 and C developing unit 430. In this control example,an example will be given in which image formation is performed in thelongitudinal direction of Japanese Industrial Standard A3-size paper.

FIGS. 6(a) to 6(d) are timing charts showing a part of sequence controloperations performed by a control section as a control means related tothis control example and FIG. 6(e) is a timing chart showing at whattiming the to-be-transferred area on the intermediate transfer belt 501passes the primary transfer section.

When the user pushes the Copy Start key, as in the foregoing ControlExample 1, the photoreceptor drum 200 and the intermediate transfer belt501 start rotating (FIG. 6(a)) and the position detecting mark on theintermediate transfer belt is detected by the optical sensor 514 (FIG.6(b)), then an F gate signal is outputted slightly later than the markdetection timing (FIG. 6(c)). First an electrostatic latent image of Bkis formed on the photoreceptor drum 200.

The revolver developing unit 400 which has stood by at the home positionrevolves 22.5° at the same timing as in Control Example 1, causing theBk developing unit 410 to move to the developing position (FIG. 6(d)).The electrostatic latent image of Bk formed on the photoreceptor drum200 is developed by the Bk developing unit 410 and the resulting tonerimage is primarily transferred onto the intermediate transfer belt 501in the primary transfer section.

After the primary transfer of the Bk toner image it is necessary thatthe revolver developing unit 400 be revolved 247.5° to let the Mdeveloping unit 440 for the next development move to the developmentstand-by position. In this case, however, if the revolver developingunit 400 is revolved 247.5° at a time, C toner stain will adhere to theto-be-transferred area on the intermediate transfer belt 501 in relationto the revolving speed of the revolver developing unit although Y tonerstain does not adhere to the to-be-transferred area on the intermediatetransfer belt 501.

In this control example, therefore, the revolution of the revolverdeveloping unit 400 for moving the M developing unit 440 to thedevelopment stand-by position is performed in two stages as shown inFIG. 6(d). More specifically, after the development by the Bk developingunit 410 is over, the revolver developing unit 400 is revolved only157.5°, causing the Y developing unit 420 to once pass the developingposition and the C developing unit 430 to stand by at the developmentstand-by position. With this revolution, Y toner stain deposited on thephotoreceptor drum 200 adheres to a not-to-be-transferred area on theintermediate transfer belt 501 (FIG. 6(e)). Then, at a timing of alittle less than one-round-rotation of the intermediate transfer belt501 after the start of the above revolution, the revolver developingunit 400 is again revolved 90°. To be more specific, the revolverdeveloping unit 400 is revolved 90° at a timing of (565.5 [mm]-40[mm]/156 [mm/sec]) elapsed from the start of revolution which is formoving the C developing unit 430 to the development stand-by position.

In case of merely starting the next 90° revolution at a timing ofone-round-rotation of the intermediate transfer belt after the start ofthe 157.5° revolution, the revolution may be started at a timingdetermined by dividing the circumferential length (565.5 mm) of theintermediate transfer belt 501 by a surface moving speed (156 mm/sec) ofthe intermediate transfer belt. In this connection, although Y tonerstain adheres onto the photoreceptor drum 200 upon 90° revolution afterthe start of the 157.5° revolution, C toner stain adheres onto thephotoreceptor drum 200 upon 22.5° revolution after the start of the 90°revolution. Therefore, for allowing C toner stain to adhere to the sameposition on the intermediate transfer belt 501 as that of Y toner stain,it is necessary to correct an error of the time from the revolutionstart until the deposition of tone stain on the photoreceptor drum 200.For correcting this error, that is, for correcting the time required for90° revolution of the revolver developing unit 400, i.e., 320 msec×156mm/sec×(90°-22.5°)/90°, the revolution is started about 40 [mm]/156[mm/sec] earlier than the time required for one-round rotation of theintermediate transfer belt 501.

By thus performing the revolution in two stages, C toner stain adheresto almost the same position as the Y toner stain-adhered position on theintermediate transfer belt 501. That is, C toner stain adheres to anot-to-be-transferred area on the intermediate transfer belt 501. Insynchronism with the arrival timing of the electrostatic latent image ofM at the developing position the revolver developing unit 400 revolves22.5°, causing the M developing unit 420 to move to the developingposition to develop the electrostatic latent image of M. In this controlexample, at the initial mark detection timing after turning OFF of the Fgate signal of Bk there is not performed an optical write for forming anelectrostatic latent image of M, but in synchronism with the next markdetection timing there is performed an optical write of an electrostaticlatent image of M.

CONTROL EXAMPLE 4

Next, in connection with performing image formation by using only Bkdeveloping unit 410 and M developing unit 440 as in the above ControlExample 3, a control example (“Control Example 4” hereinafter) will begiven below in which an image in the transverse direction of JapaneseIndustrial Standard A4-size paper is formed by double-sheet imageformation onto the intermediate transfer belt 501.

FIGS. 7(a) to 7(d) are timing charts showing a part of sequence controloperations performed by a control section as a control means related tothis control example and FIG. 7(e) is a timing chart showing at whattiming the to-be-transferred area on the intermediate transfer belt 501passes the primary transfer section.

In this control example, the revolver developing unit 400 which hasstood by at the home position revolves 22.5° at the same timing as inthe foregoing Control Example 1, causing the Bk developer 410 to move tothe developing position (FIG. 7(d)). An electrostatic latent image of Bkformed on the photoreceptor drum 200 is developed by the Bk developingunit 410 and the resulting toner image is primarily transferred onto theintermediate transfer belt 501 in the primary transfer section.

After the primary transfer of the Bk toner image it is necessary thatthe revolver developing unit 400 be revolved 247.5° to let the Mdeveloping unit 440 for the next development move to the developmentstand-by position. In this control example, however, since toner imagesfor two images are formed at a time on the intermediate transfer belt501, most of the belt surface becomes a to-be-transferred area; in otherwords, a not-to-be-transferred area becomes very narrow. Therefore, itis necessary to make some improvement for the control so as to preventadhesion of color toner stains to the to-be-transferred area on theintermediate transfer belt 501.

In this control example, after the development by the Bk developing unit410 is over, the revolver developing unit 400 is revolved by only157.5°, causing only the Y developing unit 420 to once pass thedeveloping position, while allowing the C developing unit 430 to standby at the development stand-by position. With this revolution, Y tonerstain adhered onto the photoreceptor drum 200 comes to adhere to anot-to-be-transferred area which lies behind the to-be-transferred area(toner image of Bk2) of the second sheet on the intermediate transferbelt 501 and before the to-be-transferred area (toner image of Bk1) ofthe first sheet (FIG. 7(e)). Then, the revolver developing unit 400 isrevolved 90° at a timing of half-a-round rotation of the intermediatetransfer belt 501, that is, upon lapse of (565.5 [mm]/2)/156 [mm/sec],after the start of the above 157.5° revolution. With this revolution, Ctoner stain adhered onto the photoreceptor drum 200 comes to adhere to anot-to-be-transferred area which lies behind the to-be-transferred areaof the first sheet on the intermediate transfer belt 501 and before theto-be-transferred area of the second sheet (FIG. 7(e)).

When the M developing unit 440 has thus been allowed to stand by at thedevelopment stand-by position, the revolver developing unit 400 is thenrevolved 90° at a half-a-round rotation of the intermediate transferbelt 501, that is, upon lapse of (565.5 [mm]/2)/156 [mm/sec], after thestart of the aforesaid 90° revolution. With this revolution, the Mdeveloping unit 440 moves to the developing position. M toner staindeposited at the end of movement of the M developing unit 440 adheres toa not-to-be-transferred area which lies behind the to-be-transferredarea of the second sheet on the intermediate transfer belt 501 andbefore the to-be-transferred area of the first sheet FIG. 7(e)). The Mdeveloping unit 420 develops an electrostatic latent image of M arrivingat the developing position. In this control example, an optical writefor forming an electrostatic latent image of M is not performed at thefirst mark detection timing after turning OFF an F gate signal of Bk,but is performed at the next mark detection timing.

CONTROL EXAMPLE 5

In connection with the case where image formation is performed usingonly Bk developing unit 410 and M developing unit 440 as in the aboveControl Examples 3 and 4, a description will be given below about acontrol example (“Control Example 5” hereinafter) in which there isconducted image formation in the longitudinal direction of JapaneseIndustrial Standard A4-size paper.

FIGS. 8(a) to 8(d) are timing charts showing a part of sequence controloperations performed by a control section as a control means related tothis embodiment and FIG. 8(e) is a timing chart showing at what timingthe to-be-transferred area on the intermediate transfer belt 501 passesthe primary transfer section.

In this control example, the revolver developing unit 400 which hasstood by at the home position revolves 22.5° at the same timing as inthe foregoing Control Example 1, causing the Bk developer 410 to move tothe developing position (FIG. 8(d)). Then, an electrostatic latent imageof Bk formed on the photoreceptor drum 200 is developed by the Bkdeveloping unit 410 and the resulting toner image is primarilytransferred onto the intermediate transfer belt 501 in the primarytransfer section.

After the primary transfer of the Bk toner image it is necessary thatthe revolver developing unit 400 be revolved 270° to let the Mdeveloping unit 440 for the next development move to the developingposition. In the process of forming an image in the longitudinaldirection of A4-size paper, a not-to-be-transferred area on theintermediate transfer belt 501 is wider than that in the foregoingControl Examples 3 and 4. In this control example, therefore, after thedevelopment by the Bk developing unit 410 is over, the revolverdeveloping unit 400 is revolved 270° at a time, causing the M developingunit 440 to move to the developing position at a time. Even if thenot-to-be-transferred area is wide, there is not enough time forallowing the M developing unit 440 to once stand by at the developmentstand-by position. Therefore, without a temporary stop of the Mdeveloping unit 440 at the development stand-by position, there is madeswitching from the Bk developing unit 410 to the M developing unit 440.

The M developing unit 440 which has thus moved to the developingposition develops an electrostatic latent image of M arriving at thedeveloping position. In this control example there is enough time forswitching to the M developing unit 440 at a time after the completion ofdevelopment of an electrostatic latent image of Bk until arrival of theelectrostatic latent image of M at the developing position, so that anoptical write for forming an electrostatic latent image of M isperformed at the first mark detection timing after turning OFF an F gatesignal of Bk. Therefore, unlike the foregoing Control Examples 3 and 4,the intermediate transfer belt 501 which carries the Bk toner image isnot required to idle-run before the primary transfer of M toner image.Thus, the time required for this image forming process is shorter thanthat in the foregoing Control Examples 3 and 4 and hence it is possibleto enhance CPM (Copy Per Minutes).

The developing unit switching control methods for the revolverdeveloping unit 400 described in the above control examples are storedin a predetermined storage medium provided in the copying machine 1. Inaccordance with image size and copy mode selected by the user, each ofthe above control sections which can function as a control method readmeans reads out an appropriate control method from the aforesaid storagemedium and executes the selected control method.

Thus, in each of the above control examples reference has been made toan example of a developing unit switching control method for therevolver developing unit 400 with respect to different lengths ofnot-to-be-transferred areas on the intermediate transfer belt 501 anddifferent types of developing units used. But no limitation is madethereto. Even for other image sizes and developing unit types than thosereferred to in the above control examples, there can be attained thesame effects as above by selecting an appropriate developing unitswitching control method for the revolver developing unit 400.

Although in this embodiment a description has been given of the revolverdeveloping unit 400 having developing units of four colors Bk, Y, C, andM the present invention is also applicable to any other developingapparatus if only it has plural colors of developing units.

Although the revolver developing unit 400 adopted in this embodimentuses a two-component dry developer for development, the presentinvention is also applicable to a revolver developing unit using a wetdeveloper for development. This is also true of a one-componentdeveloper. Moreover, the present invention is applicable not only to arevolver developing apparatus but also to a slide type developingapparatus.

As set forth above, according to the invention referred to in theforegoing first to sixth aspects it is possible to prevent toner staindeposited on an image bearing member from adhering to ato-be-transferred area on the intermediate transfer member and hencepossible to prevent the deterioration of image quality. In addition tosuch an excellent effect there also can be attained an outstandingeffect such that the period of time during which the developer containedin a developing unit contacts an image bearing member until actualdevelopment becomes shorter than that in case of toner stain beingadhered to the to-be-transferred area, thus making it possible tosuppress a wasteful consumption of toner.

Particularly, according to the invention defined in the foregoing thirdand fourth aspects there is attained an excellent effect such that evenif the length of a not-to-be-transferred area in the surface movementdirection of the intermediate transfer member changes depending on animage forming condition adopted in the image forming process concerned,it is possible to make switching to an appropriate control method.

Moreover, according to the invention referred to in the foregoing fifthand sixth aspects there is attained an excellent effect such that evenin case of moving a developing unit concerned to the developing positionwhile skipping over developing units not to be used in accordance with acopy mode selected by the user, it is possible to make switching to anappropriate control method.

Further, according to the invention referred to in the foregoing fourthand sixth aspects there is attained such an excellent effect as acontrol method switching operation can be done efficiently, becausethere are utilized control methods which are provided in advance.

1. An image forming system comprising: an image bearing member; anintermediate transfer member which is kept in contact with said imagebearing member; a developing apparatus having a plurality of developingunits, said developing apparatus causing a developer contained in apredetermined one of said developing units into contact with said imagebearing member to develop a latent image formed on the image bearingmember; a developing unit moving means capable of moving saidpredetermined developing unit to a developing position where thedeveloper in the predetermined developing unit comes into contact withsaid image bearing member, latent images formed on said image bearingmember being developed respectively by the developing units of saiddeveloping apparatus, toner images thus formed on the image bearingmember being primarily transferred onto said intermediate transfermember in a primary transfer section in which the image bearing memberand the intermediate transfer member are in contact with each other, andthe toner images thus primarily transferred onto the intermediatetransfer member being together transferred secondarily onto a transfermedium; and a control means which controls said developing unit movingmeans in such a manner that an area on said image bearing member atwhich the developer contained in said predetermined developing unitcontacts said image bearing member to effect development, comes tocontact a not-to-be-transferred area on said intermediate transfermember in said primary transfer section.
 2. An image forming systemcomprising: an image bearing member; an intermediate transfer memberwhich is kept in contact with said image bearing member; a developingapparatus having a plurality of developing units, said developingapparatus causing a developer contained in a predetermined one of saiddeveloping units into contact with said image bearing member to developa latent image formed on the image bearing member; a developing unitmoving means capable of moving said predetermined developing unit to adeveloping position where the developer in the predetermined developingunit comes into contact with said image bearing member, latent imagesformed on said image bearing member being developed respectively by thedeveloping units of said developing apparatus, toner images thus formedon the image bearing member being primarily transferred onto saidintermediate transfer member in a primary transfer section in which theimage bearing member and the intermediate transfer member are in contactwith each other, and the toner images thus primarily transferred ontothe intermediate transfer member being together transferred secondarilyonto a transfer medium; and a control means which, when moving saidpredetermined developing unit to effect development to said developingposition so that any of the other developing units positioned on anupstream side in a developing unit moving direction of the developingposition with respect to said predetermined developing unit passes thedeveloping position, controls said developing unit moving means in sucha manner that an area on said image bearing member at which a developercontained in said any of the other developing units contacts said imagebearing member when said any of the other developing units passes thedeveloping position, is an area on the image bearing member which areacomes into contact with a not-to-be-transferred area on saidintermediate transfer member in said primary transfer section.
 3. Animage forming system as claimed in claim 2, further comprising a controlswitching means which switches from one method to another forcontrolling said developing unit moving means in accordance with imageforming conditions involving different lengths of saidnot-to-be-transferred area in a surface movement direction of saidintermediate transfer member, and wherein said control means controlssaid developing unit moving means in accordance with the control methodswitched by said control switching means.
 4. An image forming system asclaimed in claim 3, wherein said control switching means has a storagemedium which stores a plurality of control methods correspondingrespectively to said image forming conditions and also has a controlmethod read means for reading from said storage medium a control methodcorresponding to an image forming condition for an image forming processcarried out by the image forming system.
 5. An image forming system asclaimed in claim 2, further comprising a control switching means forswitching from one control method to another to control said developingunit moving means in accordance with a type of a developing unit used inan image forming process carried out by the image forming system, andwherein said control means controls said developing unit moving means inaccordance with the control method switched by said control switchingmeans.
 6. An image forming system as claimed in claim 5, wherein saidcontrol switching means has a storage medium which stores a plurality ofcontrol methods corresponding respectively to the types of thedeveloping units used in said image forming process and also has acontrol method read means for reading the control methods correspondingto the types of the developing units from said storage medium.
 7. Animage forming system comprising: an image bearing member; anintermediate transfer member which is kept in contact with said imagebearing member; a developing apparatus having a plurality of developingunits, said developing apparatus causing a developer contained in apredetermined one of said developing units to be supplied to said imagebearing member to develop a latent image formed on the image bearingmember; a developing unit moving means capable of moving saidpredetermined developing unit to a developing position where thedeveloper in the predetermined developing unit is supplied to said imagebearing member, latent images formed on said image bearing member beingdeveloped respectively by the developing units of said developingapparatus, toner images thus formed on the image bearing member beingprimarily transferred onto said intermediate transfer member in aprimary transfer section in which the image bearing member and theintermediate transfer member are in contact with each other, and thetoner images thus primarily transferred onto the intermediate transfermember being together transferred secondarily onto a transfer medium;and a control means which controls said developing unit moving means insuch a manner that an area on said image bearing member at which thedeveloper contained in said predetermined developing unit is supplied tosaid image bearing member to effect development, comes to contact anot-to-be-transferred area on said intermediate transfer member in saidprimary transfer section.
 8. An image forming system comprising: animage bearing member; an intermediate transfer member which is kept incontact with said image bearing member; a developing apparatus having aplurality of developing units, said developing apparatus supplying adeveloper contained in a predetermined one of said developing units tosaid image bearing member to develop a latent image formed on the imagebearing member; a developing unit moving means capable of moving saidpredetermined developing unit to a developing position where thedeveloper in the predetermined developing unit is supplied to said imagebearing member, latent images formed on said image bearing member beingdeveloped respectively by the developing units of said developingapparatus, toner images thus formed on the image bearing member beingprimarily transferred onto said intermediate transfer member in aprimary transfer section in which the image bearing member and theintermediate transfer member are in contact with each other, and thetoner images thus primarily transferred onto the intermediate transfermember being together transferred secondarily onto a transfer medium;and a control means which, when moving said predetermined developingunit to effect development to said developing position so that any ofthe other developing units positioned on an upstream side in adeveloping unit moving direction of the developing position with respectto said predetermined developing unit passes the developing position,controls said developing unit moving means in such a manner that an areaon said image bearing member at which a developer contained in said anyof the other developing units is supplied to said image bearing memberwhen said any of the other developing units passes the developingposition, is an area on the image bearing member which area comes intocontact with a not-to-be-transferred area on said intermediate transfermember in said primary transfer section.
 9. An image forming system asclaimed in claim 8, further comprising a control switching means whichswitches from one method to another for controlling said developing unitmoving means in accordance with image forming conditions involvingdifferent lengths of said not-to-be-transferred area in a surfacemovement direction of said intermediate transfer member, and whereinsaid control means controls said developing unit moving means inaccordance with the control method switched by said control switchingmeans.
 10. An image forming system as claimed in claim 9, wherein saidcontrol switching means has a storage medium which stores a plurality ofcontrol methods corresponding respectively to said image formingconditions and also has a control method read means for reading fromsaid storage medium a control method corresponding to an image formingcondition for an image forming process carried out by the image formingsystem.
 11. An image forming system as claimed in claim 8, furthercomprising a control switching means for switching from one controlmethod to another to control said developing unit moving means inaccordance with a type of a developing unit used in an image formingprocess carried out by the image forming system, and wherein saidcontrol means controls said developing unit moving means in accordancewith the control method switched by said control switching means.
 12. Animage forming system as claimed in claim 11, wherein said controlswitching means has a storage medium which stores a plurality of controlmethods corresponding respectively to the types of the developing unitsused in said image forming process and also has a control method readmeans for reading the control methods corresponding to the types of thedeveloping units from said storage medium.